Correct use of "его, ее" after preposition?

Question

Here are a few sentences, for which I do not understand why "его" и "ее" do not have the letter "Н" before them.

1. "... для этого нам надо играть по ее правилам."
2. "На его месте должен был быть я."

So why don't these take the letter "н"—нём, ней? and why aren't they in the same case as the next word?

thanks))

Answer 1

Fairly easy here. Его and её only take initial «н» as pronouns in Genitive case (and, well, Accusative, because it's the same). When you have possessive adjective pronouns «его» and «её», no «н» is required.

Though, of course, these possessive adjectives (его, её, их) originate from the Genitive case of the corresponding pronouns.

As a bonus, you can consider the initial «н» a sure sign that the preposition really refers to the pronoun (remember that not all preposition require it!) :

• Я от неё писем не получал. → «от» refers to some «она» ("I haven't received any letters from her")
• Я от её писем уже устал. → «от» is a preposition before «писем», and «её» is a possessive adjective that got between them ("I am tired of her letters already").

Answer 2

Нём (себе/мне/ней/них/нас) -- is pronoun in place of a noun. Hence, it has the meanings of the noun in lexical and grammatical sense.

Его (свой/моё/её/их/наше) -- is purely possessive pronoun, as the attribute to the marking who else's.

  для   этого   нам    надо  играть  по  ее  правилам
  for   this  we_have  must  to_play by  her  rules 
  "that is what we must play by her rules"
                   or 
  "for that we must play by her rules"



   На  его  месте  должен  был  быть  я   
   On  his  place   must   was  to_be I
   "I ought myself to be on his place."


   Our   teacher   is   teach-ing   Russian     to  us.
   Наш   учитель    •   учить-щий  русский-ому  •   наc.
   "Наш учитель учит нас русскому (языку)"